texts: If someone was above the galaxy, how much of it can he see?

texts: If someone was far enough above the milky way to see the entire diameter, how much can he actually see with naked eye? I'm aware that on Earth, we only see stars within 1,000 light-years (roughly) by naked eyes. I found myself wondering if someone is more than 1,000 light years away from milky way then we won't be able to see it?


texts: We are sure that there are no brown dwarfs closer than ______

texts:
I don't quite know how to word this question.

How close could a brown dwarf be to us and have gone undetected until now?

Or maybe, with what probability are we sure that there is no brown dwarf within X light years of the sun? (For different values of X)

(Don't worry about addressing Nemesis or a hypothetical binary partner for the sun. I'm just asking about undetected brown dwarfs..., even outside of the Oort cloud but within Alpha C. I don't know how sensitive the WISE survey was.)


texts: Why is the discovery of merging neutron stars important?

texts:
I'm fairly certain people here will have heard about it, already, but apparently, two supernova leftovers clashed some 130 million years ago and some billion billion kilometres away ...

What I haven't heard yet, however, is why we should care.

I mean sure, it's an interesting phenomenon and measuring it can't have been easy.

But now that we've heard it ... what changes?

I'll admit it, I don't know particularly much about astronomy, but I'm curious:

What's the significance of having achieved this? Why does it matter whether or not we know?


texts: My question isn't why it doesn't have a circular orbit, and why an elliptical, my question is why do they move at all? What caused the debris in space to apply such a force on the planets that it has been circling around the Sun billions of years.


texts:
In the review paper on protoclusters, Roderik Overzier (pg 25) mentions the concept of virial shock heating which seems to be a process involved with galaxy cluster formation.

I'm not fully certain what a virialized galaxy cluster is apart from the fact that it's in hydrostatic equilibrium, and is one of the key distinguishing features between a protocluster and a fully formed galaxy cluster.

I vaguely understand that shock heating relates to the heating that occurs when something is travelling at supersonic speed within a medium and produces a shock.


texts: Estimating the angle covered by the star trails and deducing how long the exposure lasted

texts:
How would you estimate the angle covered by the star trails and deduce how long the exposure lasted in the following image?

Here is the photo's caption:

  Star trails beyond the Gemini Observatory on Mauna Kea, from which a laser beam forms a guide star on the Earth's ionosphere that enables the use of adaptive optics. In this long exposure, the laser beam tracks across the sky, making the visible arc. (The object in the sky at which the laser was pointed changed several times during the night.) Note that stars rise and set at an angle (not perpendicular) relative to the horizon, because Hawaii is about 20 degrees north of Earth's equator. Also, the farther a star is from the pole, the more its path looks straight, over a short distance.

I read the answer to this question about how to measure star trail angles

  The only star trail angles I'm familiar with deal with astrophotography.
  You set up a camera, put the film in, open the shutter for a time exposure of the stars. As the stars appear to revolve around the earth, they make "arcs" on the film.

  To calculate the angle of the arc, use the formula: 1 minute of time = 15 minutes of arc. If the answer is over 60, divide by 60 to get degrees of arc. So: 12 minute exposure = 180 minutes of arc or 3˚ of arc length for your star trail.

I'm not sure how to approach this problem since the answer uses time to calculate the angle of the arc, but no time is given here.


texts: Is there a place where one can see the beginning of the universe?

texts: If the night sky Is dark because we’re seeing the world as it was a long time ago, when there wasn’t much to see, is there a place on the universe where we can see the universe at the  beginning of time? Where we can look at light from the moment of the Big Bang ?


texts: How do scientists determine if a star's color is due to Doppler shift vs the star's composition vs temperature?

texts:
When looking at visible light coming from, say a star, scientists can determine by the shifting of the light towards the red or the blue end of the spectrum to determine if it is moving towards or away from earth. But they can also look at the light as it falls on the visible spectrum to determine the composition of said star. How do they determine if the light landing on the red end of the spectrum, for example, means it's moving away vs the composition of the star? How do they tell the difference?

In addition, how is all this reconciled with the fact that a star's color is also based on it's temperature?

Note: Question was expanded to include temperature after comments and updated answer subsequent to my original question asking only about motion vs composition.


texts:
We have a star having 1.8 solar mass. Justify any four means (measurable/ observed properties) by which you can identify that the object is a Pulsar and not a white dwarf or a Black Hole.

So one obvious property would be the mass. Since mass > 1.4 solar mass, we can say it is not a white dwarf.

Also the mass is less than 3 solar mass, hence its not a black hole (I'm speaking loosely over here).

Also from periodic radiation patterns, we can say that the object might be a pulsar.

I need help with 2 other means to categorize this object.


texts: Historical lunar occultation of Venus seen from Sicily in the winter?

texts:
I am looking for dates in history, from 400 years ago or older, when there was an occultation of Venus by the Moon during Winter as seen from Sicily.  Venus was rising, and according to the text that I am trying to fact check, it is not clear what phase the moon is in. The text from Shakespeare reads there were "nine changes to the watery star"  This is interpreted to mean the moon, as it effects the tides.  So the moon should be 9 days away from a full moon.

Or if an occultation is too rare would there be an appulse that matches the criteria?

1) Venus is rising.
2) The moon is waning nine days.
3) As seen from Sicily.

I am led to understand that this is a rare event, happening once every 200 years or more, and I would be curious to know about any other unusual astronomical events that occurred within +/- 23 or 24 years of this rare event.

Where could I find this information?  Is there a star chart I could check?


texts: Did big bang happen only one time or it might happen again somewhere million time away from our observable universe?

texts:
We know, everything is in existence due to big bang from singularity. Time also started from that big bang point.

So, did big bang happen only once or it might happen again somewhere million time away from our observable universe ?


texts: How are black holes doors to other universes?

texts: I'm watching MIT OpenCourseWare lectures on general relativity and not too long into the first lecture the professor stated that the Kerr Black Hole solution allowed for travel between universes. How is that possible to know? How does one derive this/come to this conclusion?


texts: What's the difference between a Newtonian telescope and a regular telescope

texts:
So, recently I was shopping for telescopes and I decided on the following telescope:

https://www.amazon.com/Celestron-21049-127EQ-PowerSeeker-Telescope/dp/B0007UQNKY/

I had exactly $180 to spend, and that telescope fit the bill. However, I also noticed that it had a variety of options in sizes, mainly the following:

60MM EQ Refractor
70MM EQ Refractor
80MM EQ Refractor
114MM EQ Reflector
127EQ Newtonian Reflector

What is the difference between all these sizes/types, how does a "Newtonian Reflector" act differently than a "regular" (for lack of a better term) reflector and what are the benefits/drawbacks?

Bonus Points: Did I choose the "highest-quality" option for the price?

Side-by-side comparison of all sizes:


texts: How much greater is the star density in our galactic bar?

texts:
How much denser is it in the galactic bar than the "normal" density at the same radius?

Is it just a few percent? or is it, say, "three times" as dense?

Or are other factors at play: star brightness, gasses?
Or do we really not know?


texts: Theoretically could the expansion of the universe affect acceleration due to gravity?

texts:
I was wondering if theoretically the universe expanding affects speed or acceleration due to gravity, compared to if space weren't accelerating.

The way I have it in my head, something is falling towards a planet with gravity G acting upon it. It's been falling for T amount of time and has gone X distance with Y left to go. Currently the universe is expanding, so the distance between its starting and ending point is a very tiny bit longer when it arrives than when it started, either affecting its distance traveled at time T, or its speed at time T.  Compare this to a non-expanding universe where the distance between start and end remains the same, no matter what point in time you choose.

I know that local gravity forces and atomic forces greatly overpower expansion forces so galaxies, solar systems, planets, and watermelons don't just fly apart. I'm just wondering if the value of either gravitational acceleration, or an object's speed could theoretically be changed slightly due to cosmic expansion.


texts: What is the current estimation for how much time the Sun will function properly?

texts: Eventually, the Sun will run out of fuel, go dark, and other bad things will happen. About how much long is it predicted that we will have a sun functioning as it is currently?


texts: Does near light-speed travel doppler-shift the light from the target into gamma rays?

texts: If a space ship travels at near the speed of light towards a star, wouldn't the light from the star doppler shift into gamma rays (or worse?) and become lethal to life and destructive to space ship structure?


texts: A moon in eccentric orbit dipping below Roche limit

texts:
Let's imagine (or do we know one?) a moon in a very eccentric orbit around a planet, with periapsis below the Roche Limit. How would it behave? Would it break apart when speeding past the planet, then reform back as it spends a long time near the apogee, as the debris and parts are pulled back together by gravity over and over until tidal forces circularize its orbit enough that it stabilizes, or is one of these states considerably dominant, and it would either never get to break apart in the short periods near periapsis, or maybe gradually break into a ring and never get to reform?

Bonus question: what considerations / problems would exploration of such a body (artificial satellites, landers, rovers, maybe even a base) face comparing to similar endeavors involving a typical moon?


texts: If a captured moon has prograde, circular and equatorial orbit, can we know it is captured?

texts: As we know, Triton has retrograde orbit , which is an stong evidence it is captured by Neptune. Suppose if a moon is captured, but results in prograde orbit, and the orbit becomes circular and lies on equator of the planet after billions of years, can we still know(or find) it is captured?


texts: How to make a telescope for viewing planets, moon and DSOs using a convex lens of aperture 100 mm and focal length 200 mm and other lenses at home?

texts:
I wanted to make a telescope with DIY things lying around in home. I read up that the aperture was a very important aspect and thus bought a convex lens with 100mm aperture and 200mm focal length, to use it as an objective lens. Apart from that, I have the following lenses lying around:-

1 convex lens    -   focal length 20 cm             aperture - 10 cm
2 convex lenses  -   focal length 17 cm             aperture - 5 cm
1 convex lens    -   focal length 2.2 cm            aperture - 2.5 cm
1 concave lens   -   focal length 8.33 cm           aperture - 5.5 cm
1 convex lens    -   focal length 200 cm            aperture - 5.5 cm

I can also buy lenses of range of Power(In Dioptres) from (-14D) - (+14D) of aperture range of 5.5 cm to 7 cm, if suggested.

I made a telescope using lens 1(as objective) and lens 3(as eyepiece) which on calculation, gave a magnification of 9x, but according to stellafane.org and http://skyandtelescope.com the minimum magnification is 14x and that I was 'wasting' the light gathered by the objective lens. This telescope showed me a bright Mars, Jupiter and Saturn, but as dots. Upon seeing the Moon, I could just see it a little bigger than what would be seen with the naked eye.

I would like to know the way to make the telescope, which I would like to use to observe planets and DSOs if possible. I am ready to make separate telescopes also, if suggested.

I would like to know which lenses should I use, or buy and the way to use it. I live in a suburb and it is not very lighted up.

Is there any way I could use the lens 1 in a telescope which I could use to view the planets?

[Edit]

I have accepted @JamesScreech 's answer and am on my way to making it. Apart from that, I would also like to use the big aperture of lens 1, for viewing DSOs, But, it has a very short focal length(just 200mm), and therefore, I am finding it hard to get any eyepiece lens which can "completely accommodate the exit pupil of my scope"(as said on skyandtelescope )

Any way to get such a lens, or make an eyepiece barrell sort of, using a combination of lenses mentioned above or buying. And how to use it?

Thanks.


texts: When was it worked out/discovered that our sun can't go supernova?

texts:
As the title says, when did we realise with reasonable confidence that our star is not going to be going out in a supernova blaze of glory?
I ask because a while ago I read The Songs of Distant Earth by Arthur C. Clarke which was from 1986 (but based on an earlier story) and a plot point is that the sun went supernova. I was suprised that this was still consdiered a possibility in the 80s but accepted it.
However today I began reading The Gods Themselves by Isaac Asimov, written in 1972, and in it a character states that the sun is too small to go supernova.
This confused me as I thought both Clarke and Asimov where both scientifically knowledgable and informed.
I can find plenty of information online about how our sun will likely end and why it won't go supernova but not any imformation on how long we have known (and therefore whether Asimov was ahead of his time or Clarke was uninformed in a way that would suprise me)


texts: How did Kepler "guess" his third law from data?

texts:
It is amazing that Kepler determined his three laws by looking at data, without a calculator and using only pen and paper. It is conceivable how he proved his laws described the data after he had already conjectured them, but what I do not understand is how he guessed them in the first place.

I will focus in particular on Kepler's third law, which states that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of the orbit.

I assume that Kepler was working with data about the planets only, plus our own moon, and the sun. I make this assumption because I don't think Kepler had data about other moons, comets, or asteroids, which had not been observed by telescope yet. If this is true, knowing that Neptune, Uranus, and Pluto were not yet discovered when Kepler was alive, this means Kepler had less than 9 data points to work with.

My friend claims that it is totally concievably how Kepler guessed this relationship (although he provides no method of how Kepler might have done it), and also that Kepler's observations are "not that hard".  As a challenge, I gave my friend a data table with one column labeled $x$, the other $y$, and 9 coordinates $(x,y)$ which fit the relationship $x^4=y^3$. I said "please find the relationship between $x$ and $y$", and as you might expect he failed to do so.

Please explain to me how in the world did Kepler guess this relationship working with so few data points. And if my assumption that the number of data points Kepler had at his disposal is small, is wrong, then I still think its quite difficult to guess this relationship without a calculator.


texts: I have a 700 mm focal and 76 mm aperture telescope. I bought it a few days ago. I tried to spot Mars for 3 days but I only see red light. I use an 20 mm eyepiece for finding it and 4 mm eyepiece for seeing but only red light is visible. Please help me.


texts: If a black hole has a mass of a universe what will be the volume of it?

texts: Will it suck the entire universe in? What will the black hole look like to us, assuming we do not immediately get sucked in?


texts:
In april 2002 the inner planets had aligned and this event was captured in this beautiful photo:

But, according to this website simulations for the date, Jupiter was pretty much away. Is that possible? Can Jupiter saw as a straight line even be so far from the other planets?


texts: Counter clockwise rotation of storms at Jupiter's north pole. What explanations have been proposed?

texts:
On Earth, high pressures and low pressures occur interchangeably. So that where two pressure systems intersect, they move the air in the same direction. But as Juno revealed the first observations of Jupiter's north pole, that doesn't seem to be the case over there. There are eight gigantic storms surrounding a ninth one, and they all move counter clockwise. With two smaller ones in between on one side that do move clockwise.
How can this be explained? How is this possible?

JPL Juno IR image of Jupiter's north pole


texts: What would it feel like to be orbiting near a merging black hole?

texts: The recent 4th discovery of gravitational waves where the black hole masses were 31 and 25 times the mass of the sun, and they released 3 solar masses of energy as gravitational waves, made me wonder what would it feel like to be orbiting this binary black hole system on a planet at 1AU when the merger happened?


texts: Has the Roche limit affect been observed having an effect on an constructed object gravitational wise to cause damage? Could the Roche limit be used to slow an object?


texts: Are black holes different sizes thanks to the different sized stars that created them?

texts: If the point in space that belongs to the collapsed star (that formed the black hole) is infinitely dense, why are black holes different sizes? Is it due to them feeding on stars and everything else that they swallow?


texts:
As we know Earth rotates around Sun and Sun around Milky Way but then Sun must have some velocity/angular momentum (I don't know that much physics terms).

So as Earth revolves around Sun, it must adjust its orbit as Sun also revolves.

Is it so? Or am I missing something?


texts: What does Ptolemy mean by "a line drawn from the centre of the earth through the centre of the moon to a point on the ecliptic"

texts:
This is from a translation of Ptolemy's Almagest

Hence it necessarily follows that the straight line draw n from the centre of the earth (which is the centre of the ecliptic) through the centre of the moon- to a point on the ecliptic, which determines the true position ([as it does] for all bodies).

I don't understand what is means when is refers to "a line drawn from the centre of the earth through the centre of the moon to a point on the ecliptic. If the moon is on the ecliptic why must this line go through it. If it is not on the ecliptic how can a straight line go through its centre to a point on the ecliptic?


texts: What if we are looking for the wrong signs of life on other planets?

texts:
I am by no means an astronomy expert, having only taken 2 semesters in college. But I am an avid follower of news reports that look for signs of life on other planets. Today it was announced that 20 new planets "with the potential for life" have been found. Along with the usual signs (temperature and water) these planets were also cited because one of them has a year of 395 Earth days, a very similar year to our own.

However, I'm always bothered by these reports because they are all predicated on the premise that life on other planets will closely resemble our own. What if life on other planets doesn't need water? What if they are not carbon-based life forms? What if they can live in extremely hot (or extremely cold) temperatures?

It just seems like a massive mistake to (effectively) say, "this planet doesn't look anything like ours, there must be no life there" or to imply that only Earth qualities (or similar) can support life. Are there any fields of astronomy that look beyond the "near-Earth" life comparisons and look for life in other (drastically different) forms?


texts:
Recently I purchase Chinese 8 inch reflector telescope from local store. When I observing starts and planets I've been having trouble getting really sharp image from this telescope. I observe Venus and Jupiter with several eyepieces but I got some "coma like" effect all the time with this telescope.

To improve the image quality I decided to collimate this telescope with my laser collimator and cheshire collimators.

Before attaching the collimation device(s) I check primary and secondary mirror alignment through draw tube and it looks like this:

But when I test this setup with laser collimator it show vast miss-alignment in both primary and secondary mirrors. After align both secondary and primary mirrors I've got view similar to this through the draw tube:

I also check above setup with Venus and few starts and I didn't notice any improvements of the image.

I try to collimate this telescope with both cheshire and laser collimator but still I didn't get any clear image with this telescope.

What was the "most likely" to have issue in this telescope?

Is problem in draw-tube assembly alignment can cause problem like this or is this happens because of the problem in primary / secondary mirror of this telescope?

Also I test this telescope with several branded and unbranded eyepieces and I didn't notice any improvement.


texts: Planets just look like dots! Beginner telescope, hoping for some advice

texts: I'm very new to astronomy and got myself a cheap, used  700mm 70mm diameter refractor scope and am able to see (what I think are) good images of the moon. However when I try to view planets I see nothing but a very small dot! Pretty sure I was looking at Mars this eve (slight red colour) but with a 10mm lens its still just a dot. I can make out no detail. Same with Jupiter. Should I be able to see detail with the specs I have? Thanks in advance.


texts: Gravitational waves and gamma ray burst: how were the error bars determined for this speed of gravity calculation? Was $H_0$ used?

texts:
This newly updated answer to How precise are the observational measurements for the speed of gravity? and this answer to How is the most accurate value of  measured? cites the November 2017 arXiv preprint Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A which says that these measurements:

constrain(s) the difference between the speed of gravity and the speed of light to be between −3×10−15 and +7×10−16 times the speed of light.

What are the major assumptions and other measurements that went into these error bars? Did they use a range of values for the Hubble constant? Was the dispersion of the interstellar medium at optical frequencies included? Were known limits to the variability of fundamental constants also applied or were those assumed to be constant? Anything else?
We don't often see uncertainties in the 10-15 range in Astronomy! :-)

Potentially related:

Have more recent LIGO/VIRGO gravitational wave measurements narrowed down the speed of gravity further?
Have there been studies of "old photons" to see just how constant things like Planck constant has been?
Are photons aged?


texts:
The three principal moments of inertia of Earth and Moon are reported, for example, in T. J. Ahrens, Global Earth Physics - A handbook of Physical Constants.

I have found reports of the moment of inertia factor $\dfrac{C}{M R^2}$, where $C$ is the polar moment of inertia, $M$ the mass and $R$ the mean radius, for planets of the solar system.

Are the three principal moments of inertia for the planets of the solar system known?


texts: How to tell whether an optical spectrum is featureless or just with a low S/N?

texts:
A featureless spectrum and a spectrum low S/N are very similar.
Is there a way to tell the difference between them?


texts: Galaxy centre X structure - could this reflect impact of black hole at galactic centre

texts:
As reported in this science article X Marks the spot Centre of Milky Way, a X shaped feature is confirmed centered on our galaxy's centre, when looking on from within the plane of the Milky Way.

In terms of the presented explanation of this feature in this article, I would think the X shape could also be explained by historical influence on star formation, and to a degree star type, of polar outflowing material from the central galactic black hole.

At least assuming, if it were true, that its rotational axis over the eons of growth has varied about the axis perpendicular to the plain of the Milky Way. I am suggesting a precession type movement over its lifetime, rather like that of earths rotational axis. The orientation of its rotational axis impacted by large mass inflows at various ties, in particular early in its life when in falling mass to black hole total mass ratio was lesser than present times.

I would think there would also be likely some gravitational interaction, like wobble, of the inner swirling mass of the interstellar material within the galactic plane as the mass of the central black hole increases non-uniformally with episodes of in-falling material, such as during earlier galaxy mergers as part of our galaxies formation process.

Outflow of material from the black hole, high energy particles of mass and radiation, would from my understanding have influence on stellar formation from changing the characteristics of the interstellar material from which the stars are later formed, as well as the effects of compression of material triggering gravitational collapse for star formation.

In terms of a precession type movement of a rotational body within a medium, the X shape seems so distinctly characteristic.  It seems to be a self evident logical possible explanation to consider. At least from a non-professional and intuitive point of view!  Yet such is not mentioned as explanation in the article.

Why should such a model be excluded as explanation for this X feature?


texts: Reading about Uranus almost 90° tilt, I was wondering if some rocky planet with mass concentration at one pole could possibly spin around its own axis, while still being locked to its star? Which means the more massive pole always points toward the star?


texts: For colonization purposes, what is so good about Titan?

texts:
I have read that Titan is the best candidate for humans to colonize in our Solar System.
However it has no water, it's atmosphere is not breathable and it is -290° F on the surface. It doesn't sound too hospitable to me. Have they forgotten that humans can't live without water?
If this is the best candidate then I think that humans are doomed. So why is Titan the best option for humans to colonize?


texts: How do I download PSRCHIVE and Tempo2 for pulsar timing on Mac Mojave?

texts:
I am fairly new to pulsar astronomy, and I need to download PSRCHIVE and then Tempo2. I have followed the instructions online here:

PulsarAstronomy.net

I'm also pretty new to linux commands/Mac terminal in general, so I think I might be missing something obvious.
So far I have untarred the tar file, which gave me a directory 'psrsoft' in Downloads.
I also attempted to open a psrsoft bash profile, but I don't think it worked because none of the psrsoft commands worked.
The instructions mention '$PSRSOFT_DIR' a lot, and I am not sure where that is supposed to be or what it represents.
Right now, my goal is to get the package installation options listed and then from there install Tempo2.

This is what psrsoft contains:

Downloads/psrsoft>bin: pkg_files, psrsoft, psrsoft-selfupdate, psrsoft-shell, psrsoft-update-index, psrsoft.py, shell_files, usr

Downloads/psrsoft>config: profile.example

Any help would be greatly appreciated.


texts: Is dark energy evenly spaced throughout Universe?

texts: Are there certain parts of the Universe that are expanding faster than others? Is there anyway to definitively measure this?


texts: Gravastar Star With no Horizon with ultracompact and very density possible?

texts:
Is that possible with some strange Quantum Effect could give rise to a completely new kind of a star? Gravastar(Gravitational Vacuum Star) an object hypothesized in astrophysics as an alternative to the black hole theory.The notion of gravastars builds on Einstein's theory of general relativity and imposes a universal "smallest size" that is known to exist according to well-accepted quantum theory. This size is known as the Planck length and is derived using the speed of light, Planck's constant and the gravitational constant. Quantum theory says that any scale smaller than the Planck length is unobservable and meaningless to physics and physicists. This limit can be imposed on the wavelength of a beam of light so as to obtain a limit of blue shift that the light can undergo. However, in the presence of an event horizon, the blue shift diverges.

In the original formulation by Mazur and Mottola, gravastars contain a central region featuring a p=-ρ false vacuum or "dark energy", a thin shell of p=ρ perfect fluid, and a true vacuum p=ρ=0 exterior. The dark energy like the behavior of the inner region prevents collapse to a singularity and the presence of the thin shell prevents the formation of an event horizon, avoiding the infinite blue shift. The inner region has thermodynamically no entropy and may be thought of as a gravitational Bose-Einstein condensate. Severe red-shifting of photons as they climb out of the gravity well would make the fluid shell also seem very cold, almost absolute zero.


texts: Convert Ecliptic to Galactic Coordinates

texts:
How to convert Ecliptic co-ordinates to Galactic? The wikipedia article on Celestial Coordinate systems contains various conversions, but this one is currently absent.

For a more specific example: I would like to convert Proper motion in ecliptic latitude and longitude from the ATNF catalog into proper motion in galactic coordinates. Using the following notation from the catalog documentation:

PMElong:     Proper motion in the ecliptic longitude direction (mas/yr)
PMElat:      Proper motion in ecliptic latitude (mas/yr)
...
ZZ:          Distance from the Galactic plane, based on Dist
XX:          X-Distance in X-Y-Z Galactic coordinate system (kpc)
YY:          Y-Distance in X-Y-Z Galactic coordinate system (kpc)

How would I calculate the following (my own addition) from PMElong & PMElat?

PMZZ:        Proper motion perpendicular to Galactic plane
PMXX:        X-direction proper motion in X-Y-Z Galactic coordinate system (kpc/yr)
PMYY:        Y-direction proper motion in X-Y-Z Galactic coordinate system (kpc/yr)


texts: How would the pocket cellular clock work?

texts:
In a museum in Lviv, I saw a pocket cellular clock. I don't have a photo, but it was a small disc that had 2 or 3 filaments in it which were pointed at the stars (one of them was Andromeda, I think). Through analyzing the position of the stars it was possible to determine the time of the night. I'm not sure if knowing the day of the year was promised.

I couldn't find the information about the way the clock was used. Does anyone know how such device was used?


texts: What is the period of Jupiter being visible in the sky?

texts:
For more than 3 months now, we can enjoy seeing Jupiter in the sky, and this will be the case until end of December.
This means that Jupiter move slowly in the sky every day, but it makes me wonder when we'll be able to see it again after December 2020?
Or, put it differently: how long does it takes for Jupiter to do a full "rotation" in the sky?
Thank you !


texts: Planets have migrated throughout the history of the solar system, and some planets could even have swapped positions. Assuming the Oort cloud does exist, has it always been at its current distance from the Sun? Or did it used to be closer? Or further away?


texts: How accurate are rederings of something entering Earth's atmosphere?

texts:
Supposedly how accurate is it when you see a movies showing something entering Earth's atmosphere traveling 1 mile to 10 miles an hour or barely at all but skinning the atmosphere they show it heating up and burning just by entering the atmosphere no speed or barely just entering?

As a second question, something coming in - say, a vehicle with rockets - to ensure that the speed does not exceed that would cause  any heating at all. What speed would that be?


texts:
Gravitational lensing is everywhere! because it falls off so slowly with $r$:
$$\Delta \phi \approx \frac{4GM}{c^2r_0}.$$
That's the first order term. For a nice derivation see Viktor Toth's The bending of light by gravity "...square rooted expression in the denominator can be simplified to first order..."
Wikipedia's gravitational lensing breaks it up into three general categories

strong lensing
weak lensing
microlensing

Questions:

What's the largest angle that light has been "seen to bend" by gravity? (of one object by a separate object)
Was it large enough that a higher order term in this expansion was at all significant?

I'm going to arbitrarily exclude seeing the accretion disk behind a black hole or other strong local phenomenon like that, so I've added "of one object by a separate object" to the title, otherwise the now famous Event Horizon Telescope image seems like a winner. (To my understanding the dimmer part across the top is light from the accretion disk behind the black hole "bent over the top and towards us" Is the angular size of the black hole in the movie "interstellar" completely overblown?) As an aside, certainly the first order equation above would no longer apply in this case.


texts: Why would all places appear to be at the center of the universe's expansion?

texts:
I just watched an iMax planetarium video that claimed that no matter where you are in the universe, it will appear that you are at the center of the universe and all things are expanding away from you.

What the heck?? Can someone please explain this?


texts:
I am researching into how coordinate systems of solar systems objects are created by reading some of the reports written by the Working Group on Cartographic Coordinates and Rotational Elements (e.g. 2009). However, I am finding it difficult to completley understand the role of time in defining reference systems.

When observing at a planet from Earth, for example Jupiter, there are a variety of factors that make it difficult to construct a reference system (including no solid surface and planetary precession), so we use geometry to define a reference system. However, because our perspective is dynamic, meaning Jupiter's surface changes and the planet's move, we say that at time J2000 we know the precise orientation and position of Earth and therefore can say from position defined at J2000 this is the coordinate reference system for Jupiter.

So, does incorporating time (e.g. J2000) mean we can say a coordinate reference system is based on the situation of an object, Jupiter in this example, at a given moment?


texts: Finding a location geographically of a celestial object, in this case the moon at one of its quarters

texts:
Years ago it was mentioned to me the New Yorker magazine had a Young Moon “contest”; who could find the youngest moon; the thinnest crescent after a new moon. I knew enough to know that I would have to know exactly when the new moon occurred and more importantly where. The Thin Crescent wouldn’t be too hard as it would follow closely the sun. Still, not knowing where the new moon happened meant that I could see anything from nothing to a one day old moon. I knew the moon traversed about 13 degrees of arc a day in so doing about 7% Of its disc was illuminated so my chance of seeing a very thin crescent depended on the new moon happening to the east of me by some 6 to 12 hours, as the “record” was something in the realm of 9hours.

So the question arose in me “Where on the globe would an observer see any of the quarterly phases exactly over their meridian?” This question led to almanacs that used hours, minutes, seconds and hundredths of seconds of measurements, all starting from the celestial spheres Greenwich or zero longitude, the Vernal Equinox, that semi-fixed(a 26,000 year cycle) starting place for celestial longitude. My question is, how can I find and use that “exact” beginning zero “longitude?

I am being hobbled here by being forced to use the very nomenclature I don’t understand. I mean I understand it, it’s simple enough, I just don’t know where to start. How do I find right ascension 18h23m52.34s on the celestial sphere? Is there a bright star at the vernal equinox that is used to fix that spot in the sky?

I begin to think I’m communicating with a machine that can’t extrapolate...no matter, as long as I get an education. Ok then here’s a question: how can I find right ascention(ra)18h 23m 52s standing out in the desert some night? I don’t need declination as the object is large and I’ll know it when I see it.


texts:
I'm interested lately in Stellarium and the functionalities which it provides especially scripting. I want to make a simulation that shows the celestial sphere with some planets like Venus, Mars, and the Pole-star.

I would make a kind of animation that starts from a departure point (France) and arrives in (Canada), but in this animation, I couldn't figure out how to synchronise between time and position (Supposing that I'm travelling to Canada at a fixed speed like in a vessel).

I've already looked for some examples in the scripts folder and I read the documentation and I found that I can use setObserverLocation() and maybe setDate() but I don't know to synchronise between both of them to make the animation realistic.


texts: Can you see both Jupiter and Saturn (and their moons) in detail during their upcoming conjunction?

texts: In December of this year, Jupiter and Saturn are going to be in a conjunction. Are they going to be close enough that you could resolve both as more than just dots and see their respective satellites in the same FOV with a telescope?


texts:
If we want to observe a galaxy for which we have optical data, what parameters we need to know to calculate its total radio flux at 3 cm?
Specially I am interested to know how to differentiate among the different process in radio, like synchroton, free-free and inverse compton? Specially when I know its Hydrogen recombination line luminosity is in the power of 42, and electron temperature 10000 K with distance 20 Mpc

From literature, I assume 3 cm is 10GHz, which should fall under either synchroton or free-free emission. But how do I know exactly?


texts:
Let say Mercury, Venus, the Moon were all in alignment between the Earth and Sun. They are all closest to each other in respective elliptical orbit.  On the other side of the Sun you have the rest of the planets.

Would their gravity combine to have any gravitational effects on the Earth?
Or are they all too far away?


texts: My family saw small dots in the sky moving rapidly at night

texts: My family and I saw small dots moving rapidly from north to south that flew in a V formation (the kind that ducks fly in). There aren't any ducks in my country. This phenomenon appeared twice. They're not stars or satellites. Could they be any type of UFO?


texts: How much of the Universe is invisible to us, and how does it affect our theories?

texts:
I watched an interview with Neil deGrasse Tyson on The Late Show with Stephen Colbert where he briefly mentioned a disturbing thought. It is something that I have never thought of before, and the idea borders on the eerie.

The idea is this (and here comes a mouthful): If all our knowledge of the Universe is based on observations of distant galaxies (and Milkyway stars), and we accept that the Universe is expanding at such an accelerating rate that at some point in the future only our nearest stars will be visible to whoever inhabits Earth, and that then those future inhabitants will need to explain the Universe based only on what they can observe, then it is possible that we are today trying to explain the Universe based on largely incomplete data.

Like, imagine that there is much more stuff out there, stuff that could help us explain some of the scientific "mysteries" such as dark energy/matter, but it is beyond our horizon. We don't know what is missing from our view.

And so I guess the question is this: How does this affect the validity of our current models of the Universe?


texts: How small a location is possible to identify when any visible star is at its zenith directly above?

texts:
How small an area on Earth is it possible to identify with any precision when any celestial object (star, planet, comet, nova) (visible with the naked eye alone) is at its zenith directly overhead?
--Would it be possible to identify an area as small as a town, or village, a square mile, or an acre, or even a building?
--How big an earthly location would be required to narrow it down if such a celestial object is directly above?
  Thanks.
PS:  I've tried to narrow the question down to get a simple to understand answer.  I'm not very technical, nor an astronomer.


texts: How do we calculate the image circle of a telescope at prime focus?

texts: I'm trying to find a suitable CCD and telescope combination. So, I want to make sure that the image circle of the telescope circumscribes the CCD. But I'm unable to find anything that tells me how to calculate the same.


texts: Why we can't see our Sun as it was a few years ago?

texts:
Our Sun is orbiting the center of the Milky Way galaxy. So many years ago, it was at a different location from where it is now.
In addition, we are observing stars and exoplanets as they were many years ago, for example stars like Sirius..

In this case, why can't we see our own Sun in its orbital path a few hundred years back.
Is there any specific reason why we can't see/observe our Sun from the past?


texts: Why does the Chandrasekhar limit affect white dwarfs differently?

texts:
The Chandrasekhar limit is the maximum mass of a stable white dwarf star. Beyond this, a carbon-oxygen white dwarf will typically explode in a type 1a supernova, due to the nuclear reactions at those temperatures.

I've heard that oxygen-neon-magnesium white dwarfs, on the other hand, will not ignite. Rather, electron capture becomes energetically favorable and they become neutron stars. If I understand correctly, this also happens for carbon-oxygen white dwarfs in binaries, if the white dwarf has most of the mass in the system.

Why is this? Can anyone walk me through, step by step, what happens if a white dwarf (of both compositions) exceeds the Chandrasekhar limit, and how each differs?

Source: https://arxiv.org/abs/astro-ph/9701225


texts: What are reliable resources for weather and all other atmospheric obstructions?

texts:
Everyone knows you need clear skies to see what the stars or the Sun is doing. Weather is not too particularly hard to determine. There are many resources out there that will tell us the weather in any city. However, they very often lack other atmospheric obstructions. Naturally, light pollution is one of the biggest concerns. But another major concern is erratic atmospheric changes due to things like forest fires.

I'm looking for good resources that will tell me not only weather, but also light pollution and other obstructions such as smoke from forest fires.

(There's a solar eclipse coming up and I'm driving 500 miles to see it.)


texts: When will the Sun end all human life on Earth?

texts: If nothing else wipes out human existence prior to this, at what point will the Sun make Earth uninhabitable for humans?


texts: How do we not know what's in the Solar System?

texts:
It seems every once in a while there's a claim of a new planet in the Solar System.

I don't understand how local planets can stay undiscovered for this long when astronomers are accomplishing what seem to be far more difficult feats.

For example, it's been a whole century since they had accurate enough measurements to tell that Mercury was precessing in a way that didn't jive with Newtonian mechanics.
Now, if there are Earth-sized planets out there, shouldn't their presence have a pretty sizable effect on the orbits of bodies such as Neptune and Pluto? How can there be such large planets with no measurable effects until 2017 technology came along?


texts: How many light years away is Earth from the closest outer edge of the black hole at the center of the Milky Way?

texts: How many light years is Earth away from the closest outer edge of the supermassive black hole in Sagittarius A* at the center of the Milky Way?  I don't mean the distance to the center of the galaxy, but to the point in the black hole's circumference where it is closest to us.


texts: I've seen several papers that discuss the Lyman-Werner Background (LWB) when modeling star and galaxy formation in the early universe. One in particular (https://arxiv.org/abs/1803.04527) even varies the strength (?) of the LWB using the units $J_{21}$. I assume the subscript is referencing the 21cm band, as opposed to a more generic use of $J_{c}$. But I am equally confused as to what the flux in terms of J is referring to. I've found and read on Lyman-Werner photons but going from these photons to a variable "background" is where I'm having a disconnect.


texts: Is the angle of the Earth's orbit on the Galactic plane changing (towards 180 degrees)?

texts: Currently the angle of Earth's orbit is aroun 60 degrees on the Galactic plane. But is this angle changing?


texts: What is the spectral reflectance of starlight in a close binary?

texts:
Ignore the Blender Monkey! (from here)

In a close-orbiting binary pair, a small but significant fraction of the light from each star falls upon the other, and the result has to be carefully modeled to simulate the observed spectrum of the system as it rotates.
One effect is that each star's radiation will non-uniformly heat the surface of the other star and alter its thermal radiation, but here I'm asking about light from one star being scattered or diffusely reflected by the other.
How is this scattered/reflected light calculated, and roughly what is the spectral reflectance?
Of course each star will be different, so one example or maybe two would be sufficient.
We often treat stars as blackbodies as an approximation, suggesting that stars have low albedo, but at some depth the electron density will be high enough so that the plasma frequency reaches visible wavelengths, increasing reflectivity, though that may be too deep to matter.


texts: How do I calculate the sun's azimuth based on zenith, hour angle, declination, and latitude?

texts:
Wikipedia cites this formula for calculating solar azimuth:

Where:

$\phi_s$ is the solar azimuth angle
$\theta_s$ is the solar zenith angle
$h$ is the hour angle, in the local solar time
$\delta$ is the current sun declination
$\Phi$ is the local latitude

I'm implementing this equation in JavaScript, but it doesn't seem correct. Can someone here help me confirm whether or not this equation is correct?

Here's my JS implementation. The only strange thing is at the end: limiting the acos domain to [-1, 1] to prevent NaN values.

const getSolarAzimuth = function(
    zenithAngle, hourAngle, declination, latitude
) {
    const cos = Math.cos;
    const sin = Math.sin;

    const cos_phi = (
        sin(declination) * cos(latitude) - cos(hourAngle) * cos(declination) *
            sin(latitude)) /
          sin(zenithAngle);

    return Math.acos(Math.min(Math.max(cos_phi, -1), 1));
};


texts: How to determine what directly imaged objects are?

texts:
A directly-imaged object found close to a certain star could be another star in the background or a  companion planet?

How would one possibly discern between the two?
Also would we know it works?


texts: Best way to simulate star sizes to scale in celestial sphere

texts:
Using the Hipparcos catalogue, I am trying to create a celestial sphere. As all the stars are a fixed distance from the centre of this sphere, the only way to differentiate the distances and magnitude is through the sizing of each model of the star.

With the Parallax angle $p$ and visual magnitude $m_{vis}$ of each star from the catalogue, I have created the code below to calculate the radius of each star relative to the radius of the sun. I had used this website to calculate this. The problem is that some stars are too big, almost bigger than the celestial sphere itself! I am trying to keep the radius below a certain threshold.

Here $p$ is in arcseconds and $d$ is in parsecs.

$$d = 1 / p$$

$$M_{abs} = M_{vis} - \log_{10}(d^5) + 5$$

$$ T_{surf} = \frac{8540}{CI + 0.865}$$

$$R_{rel} = \left( \frac{5800}{T_{surf}} \right)^2 \sqrt{(2.512)^{4.83 - M_{abs}}}$$

$$R_{Sun} = 2.5$$

$$r = R_{rel} R_{Sun}$$

    private void positionStar()
    {
        double radius;
        cartesianPositioningCalc();
        gameObject.transform.position = cartesianPositioning;

        // Convert Plx from milliarcseconds to arcseconds (seconds of arc)
        double PlxSOA = Plx / 1000;
        // Calculate distance from equation d=1/p
        // distance d is measured in parsecs and the parallax angle p is measured in arcseconds.
        double dPC = 1 / PlxSOA;

        double absMag = Vmag - math.log10(math.pow(dPC, 5)) + 5;

        surfaceTemperature = 8540 / (CI + 0.865);

        double relativeRadius = math.pow((5800 / surfaceTemperature), 2) * math.sqrt((math.pow(2.512, (4.83 - absMag))));
        double radiusSun = 2.5f;

        radius = relativeRadius * radiusSun;

        gameObject.transform.localScale = new Vector3((float)radius, (float)radius, (float)radius);
    }

First of all, am I using the right mathematics? If I am, then how can I make sure that the radius is below a certain maximum threshold (5 units, for example.)?

If the mathematics is incorrect, please assist me on how to fix it.

Thanks!

With a slightly modified version of @Mike G's formula in their answer below:

radius = 50 * math.pow(10, (-1.44 - Vmag) / 5);

I was able to get this result:

And if I'm not mistaken, I think I can locate the Ursa Major constellation offset slightly to the left from the centre of the screenshot.

However, after uploading to nova.astrometry.net the stars and constellations are still not being picked up. Is this because the star sizing is still a bit incorrect or is this issue on the website's side?

I can use another website/app such as Stellarium to cross-check, is there a way the app can process an input image or do I try to recreate using my image on the app?


texts: The Sun and Regulus together at the summer solstice

texts: Viewed from Stonehenge, when did the Sun and Regulus rise together at the summer solstice? Also when did Regulus rise before the Sun and would have been seen before the dawn glare?


texts: I often hear about planetary ring systems, and even some moons might have them, but how about stars? Can a star also have rings?


texts: How does Saturn's ring rain fit with the earlier "big empty" results?

texts:
In 2017, the results from Cassini's flight through the region between Saturn and the inner edge of the rings were described as the "Big Empty". Nevertheless in 2018, the region was described as having a huge amount of "ring rain" consisting of dust grains coated in ices and organics falling into the atmosphere.
How can these be reconciled? Was the earlier determination of the conditions in error, is this a consequence of variability or sampling different types of particles, or something else?


texts: Is there a lower limit for the altitude of orbiting objects?

texts:
As I understand it, one object can orbit another at a variety of altitudes, and the stability of the orbit is determined by (among other things) the speed of the orbiting object.  Go too slowly and you'll fall into the larger object, go too quickly and you'll fly off into space, but go at just the right speed and you'll orbit indefinitely.  And while an object could be in a stable orbit at Altitude X by going Speed A, it could also be in an equally stable orbit at Altitude Y by going Speed B.

If that's true (and given my limited understanding, feel free to say it simply isn't), is there a lower limit?  An atmosphere would obviously cause drag, but given a lack of atmosphere, could you theoretically have an asteroid stably orbiting an Earth-sized planet at an altitude of 50 feet, if it were going fast enough?

If not, why not?  What forces limit the closeness of an orbiting object?  Also, simple explanations would be most appreciated: I've heard of things like Lagrange points and such, but I don't have a good understanding of them.


texts: How long would it take a Boeing 747 to go around Jupiter?

texts: Assuming a 747 could fly around Jupiter at its top Earth speed (I know this assumption is unrealistic), how long would it take to fly around Jupiter once?


texts: Would it be possible to calculate the expected frequency of impact craters of all sizes on Earth

texts: Can the expected frequency of all sizes of impact craters on Earth be calculated based on the preservation of impact craters seen on the Moon? Taking into account the difference in gravity and atmosphere of the Earth and Moon, and assuming a similar age for both, it should be possible to come up with a reasonable estimate of how many meteorites, etc have hit the earth and their size distribution.


texts:
I have a doubt.

Can stars be observed from space by x-rays, near infrared and radio wavelengths?


texts: What good is the Einstein de-Sitter model?

texts: Why is the EdS model still taught and used to predict the age of the universe if ours is not an EdS Universe. Does this model have any pros?


texts: Why do these photometric observations of Betelgeuse look "quantized" in 0.1 magnitude steps?

texts:
The most interesting question related to this Will Kinney tweet would be what's going on with Betelgeuse? but I can't help but wonder why much but not all of the photometric data is clustered in increments of 0.1 magnitude.

These stripes were the result of posting some preliminary data where analysis had some ultimately uninteresting effect that later disappeared, but I don't think that's the case here.

  So I downloaded data on Betelgeuse from @AAVSO . Here's a plot of visual magnitude vs Julian date for the last 10 years. Clearly something very strange going on -- a whole lot of anomalous dimming happening very quickly, in the last few months.


texts: Can we find rocks from Venus or Mercury on Earth?

texts: As we know, we can find rocks from Mars on Earth, how about Venus and Mercury? Is that not found yet? Or it is impossible to find them because they are closer to the Sun, and debris won't go far away from the Sun?


texts: How many astronomers are there in the world today?

texts: How many now living individuals have a PhD in astrophysics, or by some other definition can be reasonably classified as astronomers? I think Richard Feynman once said that only one in a million is an astronomer, I would think there are many more today.


texts: Extent of knowledge of potential future gamma ray bursts

texts:
What do we know about the locations and directions of potential Gamma ray bursts that may intersect with Earth in the future, or are we mostly blind to when such an event directed our way may occur & cause a mass extinction event?
In other words, are we able to predict to any degree at all when one might occur close enough that there may be some potential impact to life or life on Earth or infrastructure?


texts: In relation to the image above, although the topic relates to math, how can I visualise the situation described in the final equation presented, i.e., the declinations, ascensions, solar and lunar radii. A diagram would be of great help.


texts: Why are accreting objects portrayed with a white flash at the point where the gas stream from the star enters the accretion disk?

texts: When I see images of accreting objects, the gas from the donor has a white flash where it meets the accretion disk, like in the image below. Why does this happen if it is true, and why is it there?


texts:
Can somebody tell me why the constellations I've marked here have areas that resemble pieces of a puzzle, with very varying width and height and shapes, instead of being logically/evenly laid out on the sky?

Virgo is much bigger than Cancer, for example.

The image is a screenshot from Stellarium.


texts: Does the Moon capture radiation pressure from the sun causing momentum from photon propulsion?

texts: Could a planet's main use for natural satellites be that of a photon sail?


texts: How many new galaxies enter the observable universe each day/year/decade?

texts:
Each day light has more time to reach our eyes from distant galaxies. In one day, light travels 2.59×10^10 km.

So our observable universe (assuming my simple math skills apply here and there isn't something about relativity to take into account), expands that much every day in all directions. So at what rate are new galaxies being encompassed by the expanding observable universe?


texts: In Chromoscope, what is the X-ray source between Ophiuchus, Libra and Scorpius?

texts:
Chromoscope is a nice toy. Pressing 'L' shows the constellations as guide.

There's a bright X-ray source between Ophiuchus, Libra and Scorpius. In Hydrogen-Alpha there are two circular formations to the right and left of it. Are the latter formed by polar jets from the former, and what is that object?


texts: How does neutron star collapse into black hole?

texts:
We know the spectacular explosions of supernovae, that when heavy enough, form black holes. The explosive emission of both electromagnetic radiation and massive amounts of matter is clearly observable and studied quite thoroughly. If the star was massive enough, the remnant will be a black hole. If it wasn't massive enough, it will be a neutron star.

Now there's another mode of creation of black holes: the neutron star captures enough matter, or two neutron stars collide, and their combined mass creates enough gravity force to cause another collapse - into a black hole.

What effects are associated with this? Is there an explosive release of some kind of radiation or particles? Is it observable? What physical processes occur in the neutrons as they are subjected to the critical increase of pressure? What is the mass of the new black hole, comparing to its neutron star of origin?


texts: How much has the apparent magnitude of the Sun changed?

texts: Did the apparent magnitude of the Sun change in the past 1000 years or so? If so, for what reasons?


texts: Heliocentric distance from galactic x y and z coordinates

texts: How can I get heliocentric distance to a star when I have galactic x y and z coordinates known ?


texts: What contributes the most to the seasonal temperature variation?

texts: The seasonal temperature is ultimately due to the precession of the Earth around the axis. But what I'm curious about is...  is it due more to the side experiencing winter being farther from the sun or is it more due to the fact that the days are shorter and the nights are longer?


texts:
I'm working on detecting optically overlapping galaxies for future astronomical surveys. I'm looking into feature extraction for HST images and have been computing the fourier transform of images in the hope to construct a metric signifying overlapping vs non-overlapping light profiles.

For example, I have the following image after having filtered out noise:

I've been having trouble trying to interpret the results after taking the fourier transform, taking the magnitude of each element, and then plotting the logarithm of the magnitude.


texts: If earth changed its orbit, what would happen to the moon?

texts:
Let there be an accelerating force acting on earth over an arbitrarily long time to change its orbit around the sun (e.g. by asteroids repeatedly passing by close to earth (intentional or not) or whatever). For this question consider only the effect of earth's gravity on the moon (and vice versa), not other influences (e.g. not the immediate effect the asteroids passing by have on the moon's orbit).

If such perturbations are weak enough yet of sufficient duration to change earth's orbit around the sun, how does it change the moon's orbit around earth? My considerations:

Since the moon remains in orbit around earth even as the earth goes around the sun there obviously is some lower limit to how fast earth can accelerate. However, that force wanders around earth once a year, hence its effect on the moon's orbit is evening out - and the effects from the sun are supposed to be ignored here, anyway, so let's have the force go in a constant direction.

The force acting on earth can be decomposed into $F_\perp$ perpendicular to and $F_=$ coplanar with the moon's orbital plane. $F_\perp$ moves earth out of the plane of the moon's current orbit, so the moon's orbit will adjust to find a new plane: if the force is continuously applied in the same direction the moon's orbit will change until $F_\perp = 0$, so this factor can be neglected.

$F_=$ will cause the moon's orbit to become very elliptical, the moon passing closer on the side toward which earth is accelerated and being further away on the other side. If the force is indefinitely sustained something will happen to the moon, but under which circumstances will which scenario happen?

the moon remains in orbit,
the moon gets tidally torn apart,
the moon crashes into earth,
the moon leaves orbit of earth (though the moon will probably say that it's earth which ran away...),
anything else interesting I forgot.


texts: Can a star orbit around multiple planets or a planet with massive moons?

texts:
Note: I'm not talking about a star orbiting around a single or lonely planet :)

I know a star orbiting a planet is almost impossible because if a planet is more massive than a star, that "planet" would probably be a star. But how about if the planet is not lonely like:

A multiple planetary system which consists of free floating planets only but the planets are so massive so that their total mass is larger than a star
A planet itself is not more massive than a star but it has so many massive moons

Is it possible for a very small star to orbit around planets in such situations?


texts: Why do Earth and moon move apart but binary black holes move closer?

texts:
According to the accepted answer on Is the moon moving further away from Earth and closer to the Sun? Why?, the moon is receding from Earth because tidal forces and friction cause energy to be lost.

However, according to LIGO's website,

  As the two masses rotate around each other, their orbital distances
  decrease

as they lose energy radiating inspiral gravitational waves.

Why do the bodies move apart in the Earth-moon case but closer together in the black hole case?

If both opposing phenomena are present but a different one is stronger in the two cases, what determines the fate of a system?


texts: I wonder what is the contribution of metals and dust locked into planets, with respect to the amount in the interstellar medium (ISM). In other words, when we measure the metallicity and the dust mass of a galaxy we are sensitive to metals and dust in the various phases of the ISM, and thus the observed value is a lower limit. Is the "correction" a second-order variation?


texts: Why does the Sun track out a seemingly sinusoidal path on the celestial sphere?

texts:
The question arose because I wanted to understand the path of the ecliptic after reading about it here: https://community.dur.ac.uk/john.lucey/users/solar_year.html

Is the sinusoidal path due to earth’s axial tilt? Or is it related to mapping the path on a 2D pane? Is there way to visualise this to aid understanding?


texts: How to correct observed flux densities for redshift

texts:
Say I have a spectrum of a galaxy at a redshift $z$, in flux density units of erg/s/cm^2/Angstrom. I'd like to recover the spectrum (in the same flux density units) at z=0, i.e. at its rest wavelengths. How do I do this?

I saw this paper (Distance measures in cosmology, Hogg (2000)) but am still confused, because of their use of luminosity and luminosity densities, and in my class I only have fluxes.


texts: Did I see a meteor(oid) or space debris or something else?

texts:
About three years ago I think, at night I've seen something enter the atmosphere in a glowing bright fireball. But I think it wasn't the fire from burning up, but because of ionization (what also happens to meteors) but I might be wrong and it may have indeed been fire. There was also a kind of a contrail behind the glowing ball (which was visible only in the vicinity of the ball). It was in November in the northern hemisphere, so it might have been a Leonid, but it didn't look like the usual meteors. It was much slower and brighter. It was cloudy that night and eventually it disappeared behind a cloud. Whatever it was, it wasn't a typical meteor nor was it an airplane (it was quite straight above me, soundless and it was something entering the atmosphere).
My guess is it was either a meteoroid that was very dense and comparably big (several feet) and a comparably slow one, or it was some retired satellite or some spacecraft debris entering the atmosphere. What do you think?
update: I estimate its speed at about 1/6 that of a typical shower meteor.


texts: How many moons in the solar system have water?

texts:
I've heard of a few moons in the solar system that have water, like europa and enceladus. However, I can't find information about whether there are other moons as well. I'm guessing there are more, but I'm not totally sure.

How many moons in the solar system have water (either liquid or frozen)?


texts: What meteor shower has the radiant that appears furthest west after sunset, and why?

texts:
When it finally gets answered, answers to How to calculate the position of a meteor shower's radiant point based on its associated comet's orbit? will help us understand why the radiant points of meteor showers tend to be closer to east than west after sunset and before midnight, but some comet orbits could potentially produce meteor showers who's radiants could be near the zenith at sunset and set in the west around midnight.
Question: What meteor shower has the radiant that appears furthest west after sunset? What is it about the associated comet's orbit that makes this so?


texts: Lunar illumination (percentage) data on an eclipse day needed

texts: I am looking for the information on the percentage illumination of the moon during the different stages of a total lunar eclipse. Is there a database from where I can acquire information of lunar illumination, distance and angle for the July 27th 2018 lunar eclipse for a given location?


texts: Pictures of galaxies, nebulae: which way is up?

texts: We are used to viewing pictures of the Earth and the planets with their North Pole pointing towards the upper portion of the photo. But when a space telescope snaps photos of cosmic entities, do they rotate the photo around to get an attractive composition? Is it a subjective rotation, or are there rules? Which way is up?


texts: The lunar mare are basins that filled up with magma over the course of the Moon's history. So there were eruptions on the surface of the Moon at some point in time. When did the last eruption happen on the Moon?


texts: How do astronomers estimate the total mass of dust in clouds and galaxies?

texts:
As I understand it astronomers estimate the mass of stars by studying the orbits of their companions (other stars or planets).  This helps them to derive relationships between the luminosities and masses of stars.  This allows them to infer the mass of stars of known luminosity.

But how do astronomers estimate the mass of cosmic dust in particular clouds or in entire galaxies?


texts: Is the delay to correlate the signal from one dish to another on a VLA radio telescope longer for red shifted objects?

texts: When observing 2 objects side by side close to the horizon with a very large array, one receding fast & red shifted, is the delay to correlate the signal from the closest dishes to the further ones longer for the red shifted object?


texts: What is the smallest diameter black hole that can be formed naturally?

texts:
I was watching the video I Put a 1mm Size Black Hole Next to Earth and This Happened - Universe Sandbox 2 discussing what would happen if a 1 millimeter black hole is orbiting the earth, and I began questioning myself, is it really possible to naturally have such a small black hole?

What is the smallest size that a black hole can form naturally in real life?


texts:
There appears to be varying positions/spots of view of other planets from Earth, if I'm not wrong. Now, given that all planets are on the same plane, why do their positions change on the night sky for similar seasons, when they come back to earth's view? How is this possible?

Shouldn't all the outer planets be observed within the horizon's range (and not in the high skies), now that they lie on the earth's tropical plane? Give or take a short distance from where the horizon 'touches' the ground?


texts: What's the correct distance to Betelgeuse?

texts:
Reading [1], [2] and this smartphone app[3], I'm getting different answers to the question.
The smartphone app is stating 427ly. [1] states approx. 700 ly,  and [2] states approx. 724ly.
Even if I take [1] as the correct value, why does [2] state 724ly and the app stating 427ly?
Thanks
[1] - https://en.wikipedia.org/wiki/Betelgeuse
[2] - https://earthsky.org/astronomy-essentials/how-far-is-betelgeuse
[3] - https://www.escapistgames.com/sc.html


texts: During night on the Moon is there Earth light and Earth phases?

texts:
I realise that night time on the Moon is two weeks, however I am curious if during the night there, whether or not the Earth reflects light in similar manner to the Moon.

On the one hand, I know it must (as seen in photographs from astronauts), but what I'm really wanting to know is:

Does the Earth have phases like the Moon and do they work in a similar manner for an observer on the Moon?

I assume the Earth isn't as reflective as the Moon since the Moon is much lighter in colour, however the Earth is also fairly larger, does this make any difference in the amount of light reflected?

Finally, if the Earth does appear in phases, does that mean that the two week long nights on the Moon are sometimes pitch black (new Earth) and sometimes less than pitch black (full Earth)?


texts: What causes a neutron star to become a pulsar?

texts: Leading on from this pulsar question, what criteria/processes make neutron stars become pulsars, do all neutron stars become pulsars?


texts:
I'm working on an animation that replicates our Solar System. I know this has been done before, but I'm working on something that requires me to make one myself. With that being said, I'm having a tough time deciphering the JPL Ephemerides. Particularly for the Moon because I have already figured out how to work the VSOP Ephemerides, which gives me the positions for the planets.

Does anyone know of a source I could use to learn how to utilize the ASCII JPL Epehemerides file that are found here? It would be a huge help and I think a step by step walk through would probably help others in the future.

Thanks!

Link to Epheremerides files


texts: Does naked eye miss a lot between local stars and distant milkiness?

texts: When I look at the night sky I seem to resolve points which are local stars and milkiness at further regions of the galaxy. But what is between? I think there must be a lot of stars which are too faint to see individually, and yet not perceivable as part of a dense region of stars. Is this at all accurate? Does anybody know how much we fail to see when we think we are looking?


texts:
This paper on Arxiv indicates that the signal period of known pulsars is 1 ms-15 s (0.07-1,000 Hz since F=1/T). I have an EE background and it's a long story how I ended up here, but suffice it to say I'm weak on astronomy.

This chart on Wikipedia implies that the entire range of pulsar frequencies should be absorbed by the atmosphere, but this guide and even this budget RF telescope writeup clearly state that pulsars can be detected from Earth's surface.

My questions is: how are pulsars detected from Earth--what am I missing here? Are RF telescopes simply that receptive? Any insights appreciated.


texts: Timing based on upper or lower part of sun/moon

texts: When you time an astronomical event, such as the moon/sun rising above the horizon, do you note the time when the the first sliver breeches the horizon or when the bottom of the moon/sun just becomes visible?


texts: Are the bright dots in this image individual stars?

texts:
I saw this picture of M51 (Whirlpool galaxy) and noticed many, many bright dots around and in the galaxy. I have freehand-circled a few as examples. Are these individual stars (probably supergiants) in the Whirlpool Galaxy, or are they clusters, stars in the Milky Way, or something else?

Source: ESA/Hubble


texts: Planning to revisit Arthur Eddington's experiment from 1919

texts:
As the fateful day draws closer, the United States will host a celestial event that comes once in three generations. I plan to witness this first hand at the exact center of greatest eclipse.

In the planning of this, I have immersed myself in the work of Eddington back in 1919, confirming Einstein’s General Theory of Relativity (and its explanations for gravity) was right.

I personally, could not just stand-by and just look at this event and not do something more. Knowing that failure, due to weather, equipment, and or other forces are stacked against me, I have to make an attempt because a chance like this will never present itself again in my lifetime.

My goal in coming here today is simple, I need some advice from someone who knows telescopes and cameras.

The equipment I plan on using is :

a 150 mm Celestron 6SE with an automatic GOTO mount, and an equatorial wedge.
The camera will be a Sony A 6000, with an adapter.

TThe field of view is large because of the 1500 mm focal of the telescope. I chose device because of a trade-off that has to be made, Eddington used a 100 mm refractor scope when conducting his experiment. With any shift in the apparent position of the stars behind the sun as Einstein predicted, the 150 mm telescope should, in theory, coupled with a camera that has a 24-megapixel sensor, have the necessary resolution to image this shift like Eddington's image. If any shift to the stars is to be captured, they will appear within close proximity surrounding the eclipse.

My problem is as follows, the telescope I will be using, will be the lens, and now the lens has only one f-stop. At f/10 and only having the ISO and shutter speed to work with, what exposure would I use ?

Knowing that a long exposure would only manage to capture the outer corona, and possibly engulfing and obscuring any stars that shift. Likewise, a shorter exposure would capture the inner corona and thus not image any stars at all.

I do however plan to bracket 5 stops (shutter speeds) to better my odd of catching any shift.

Would anyone have any other useful tips trick to further better my odds in my endeavor?

Thanks In advance

Bob Brooks


texts: How to convert a function of keplerian elements to a function of equinoctial elements?

texts:
I need to convert the Keplerian orbital elements to the equinoctial elements and then use the converted elements"equinoctial" in the function "d" below. To do this, first, I converted the Keplerian elements to equinoctial elements (a,h,k,p,q,lamda)  using the formula in the attached picture, BUT I need to know what is the equinoctial elements that correspond to each of e,i,w,omega so that I can replace each keplerian elements in the function "d" with the equinoctial element.

d= Sin[omega]* Cos[w]2 +[w] + Cos[omega] Cos[i] Sin[w - Omega]


texts: What is the air pressure in the heliosphere (Sun's atmosphere)?

texts: Specifically, I am wondering what the pressure is at a distance of around 1 AU. Does it it decrease quadratically with distance?


texts:
I look for an image that I know exists.
It is an image showing the whole sky, showing all stars and galaxies bright enough to be visible at the resolution, projected to the same shape that is commonly used for the microwave background / CMB.
I am sure the image is well known and has a name.
Can you give a reference to it?


texts: I was helping classify galaxies at Galaxy Zoo when I came across this picture of a galaxy. The dull red body looks huge compared to the galaxy. Is it because it is closer to the earth? It would be interesting to know what is going on here.


texts:
I want to draw a Sun in a 3d scene, so I figured the easiest way would be to calculate the latitude and longitude of the point perpendicular to Sun position and then just move it "up".

As a starting point, I want to calculate it's equatorial coordinates: declination and right ascension.

I followed instructions from this page, also cross-referencing with this one

Here's my code (in Swift):

import Foundation

class Astronomy {

  static func julianDays(since: Date) -> Double {
    return since.timeIntervalSince1970 / 86400 + 2440587.5
  }

  static func daysSinceJ2000(for date: Date) -> Double {
    return julianDays(since: date) - 2451545
  }

  static func sunAxialTilt(for daysSinceJ200: Double) -> Double {
    return 23.4393 - 3.563e-7 * daysSinceJ200
  }

  static func sunEquatorialPosition(on date: Date) -> (declination: Double, RA: Double, distance: Double) {
    let daysSinceGreenwichNoon = daysSinceJ2000(for: date)
    var meanLongitude = 280.460 + 0.9856474*daysSinceGreenwichNoon
    var meanAnomaly = 357.528 + 0.9856003*daysSinceGreenwichNoon
    meanLongitude = meanLongitude.rangedUpTo360
    meanAnomaly = meanAnomaly.rangedUpTo360
    let eclipticLongitude = meanLongitude + 1.915*sin(meanAnomaly.radians) + 0.020*sin(2*meanAnomaly.radians)

    let distanceToEarth = 1.00014 - 0.01671*cos(meanAnomaly.radians) - 0.00014*cos(2*meanAnomaly.radians)

    let axialTilt = sunAxialTilt(for: daysSinceGreenwichNoon)
    let rightAscension = atan2(cos(axialTilt.radians)*sin(eclipticLongitude.radians), cos(eclipticLongitude.radians))
    let declination = asin(sin(axialTilt.radians)*sin(eclipticLongitude.radians))

    return (declination: declination.degrees, RA: rightAscension.degrees, distance: distanceToEarth)
  }
}

extension Double {
  var radians: Double {
    return self * .pi / 180
  }

  var degrees: Double {
    return self  * 180 / .pi
  }

  var rangedUpTo360: Double {
    let multiples = (self/360).rounded(.down)
    return self - 360*multiples
  }
}

However, it does not seem to be working correctly. As of the time I write this article, the result is declination -16.98772292697088, RA -44.80674952684117. Compared with https://theskylive.com/sun-info the declination is about right (-17° 03’ 38”), but right ascension is way off:  20h 59m 43s, which is 314.93 degrees according to this site

What went wrong?

Edit:
As it turned out nothing was wrong, I can get to the second part: calculating the geographical point. (the title suggests it after all)

AFAIK for latitude, I can use declination as is and for longitude, I
need to calculate Greenwich Hour Angle(GHA)

After examining several articles, I found these equations:

SHA[°] = 360 – 15 · RA[h]
GHAAries[°] = 15 · GST[h]
GHA = SHA + GHAAries

For my case, I suppose I may use RA as is, without multiplying its hour component by 15. What keeps me a bit puzzled is GST:

May I use Greenwich Mean Sidereal Time instead if I don't need the calculations to be super precise?
Is there any "GST for dummies"? All the calculations I found rely on some floating second offset, I found this table but have no idea how to use it.


texts: My fiancé and I took our youngest sons out to sky watch at the river near by. The older of the 2 was looking up and said - "look it's Santa's reindeers!" All of us looked up and saw a literal straight line of what looked like hundreds of stars in a straight line, like in a formation, moving due north. They didn't come from the south, it's more like they just appeared and disappeared. Does anyone have any idea what we may have seen?


texts: Estimation of average rock and asteroid mass associated with different stars

texts:
There is a lot of information on different star types and compositions.

Can we make estimates of the total mass of rock, ice, and non stellar matter that orbits stars based on star evolution models?

Take, for example, the sun - are there studies of the total mass of non-solar material associated with the sun and it's composition regardless of accretion. What about nearby stars?

Can we estimate how much non-incandescent material the galaxy contains aside from dark matter?


texts: When and why was zero right ascension defined to be at the vernal equinox?

texts: When (and why) was zero right ascension defined to be at the vernal equinox?  Who established or defined this standard?


texts: Do integer multiples occur in astronomy data?

texts:
I'm a statistics student, currently working on "quantum inference", which is the statistical problem of inference in the quantum model:

$y_i = \alpha + \beta x_i + \epsilon_i$

which is similar to a simple linear model, except instead of $x_i$ being a known constant, it is instead a random integer. The prototypical example is von Mises's analysis of atomic weights in 1918, trying to assess the significance of the observation that they tended to be integer multiples of the mass of hydrogen. For more information on the subject see DG Kendall's overview.

I am not sure if I should continue working on this problem because the applications are so few, mostly in archaeology, where weights or lengths are often integer multiples of an ancient unit of measure. However, it occurred to me that there may be applications in astronomy.

For example, perhaps the brightness of a spot has been measured, and the spot could represent any number of objects which are not distinguishable at the present resolution. This could be a problem of quantum inference... but only if if were for some reason known a priori that all objects had to have the same brightness.

So, while that specific scenario might be implausible, is there some problem in astronomy data analysis that involves quantum inference in this sense? In other words, are there ever cases where a histogram of the data shows multiple, equally spaced modes?


texts: When we say the sun takes 230M years to orbit the galaxy, what is this in relation to? We measure the earth's rotation relative to the distant stars. What is the reference for our Sun's motion around the galaxy?


texts: Where to find spectra from Las Campanas Redshift Survey?

texts: I am interested in the optical spectrum of a galaxy. In SIMBAD the reference for the redshift of my galaxy is the Las Campanas Redshift Survey. Hence I am looking for the optical spectrum from which the redshift was estimated.


texts: Why are we building larger land-based telescopes instead of launching larger ones into space?

texts:
This question is a follow-up to Do bigger telescopes equal better results?

How much bigger does a ground-based mirror have to be to match what a space-based one can do?  I guess I'm asking primarily for visible light, but I'm interested in general too.

I guess on the ground, you're safe from micrometeorites, so it will probably last longer.  At what point does it become cheaper to build a telescope on the moon or something?


texts: Why do planets rotate and revolve in the universe?


texts: Finding the lens diameter of a telescope by magnitude

texts:
We have a telescope that can see stars with about +14 magnitude. how to find its lens (or mirror) diameter? (I mean the $D$ in formula $\theta = 1.22 \lambda / D $)

I don't know whether I can assume that the temperature of star is equal to temperature of the sun or not. but if we think that they are equal we get this:

$m_{\mathrm{sun}} - m_{\mathrm{star}} = -2.5 \log_{10} (\frac{b_{\mathrm{sun}}}{b_{\mathrm{star}}}) = -2.5 \log (\frac{\sigma t^4 \times 4 \pi R_{\mathrm{sun}}^2 /(4 \pi r^2) }{\sigma t^4 \times 4\pi R_{\mathrm{star}}^2 / (4\pi r_{\mathrm{star}}^2)})$ if we solve this with what we know about magnitude of sun (-26.83 or something close to it between -25 to -27) and another things like radii we get:

($R/r = 3.16605 \times 10^{-11} \mathrm{rad} = \tan \theta/2 = \theta/2$ and from that and formula
$\theta = 1.22 \lambda / D $ and $\lambda = 550 nm$ we get something like D = $10596.8 \mathrm{m}$. which is very big for a telescope. what's the problem? how to solve this? Is this answer correct?


texts: Do Electric Charges in the Van Allen Radiation belt move in Opposite Directions?

texts:
The Wikipedia Article on Van Allen Radiation Belt gives the following explanation:

  A Van Allen radiation belt is a zone of energetic charged particles, most of which originate from the solar wind, that are captured by and held around a planet by that planet's magnetic field.

The term "energetic charged particles" hinted to me that these charges are in motion and in very large velocities.

Let us assume, a small portion of space in the Van Allen Radiation Belt, compared to the size of Earth. We can assume the magnetic field in this space to be uniform due to our previous assumption. From the formula for finding Magnetic force on moving charges, $$\vec{F}=q(\vec{v}\times\vec{B})$$ where $\vec{F}$ is the magnetic force on the charged particle, $q$ is the electric charge of the particle with an appropriate sign, $\vec{v}$ is the velocity vector and $\vec{B}$ is the magnetic field vector.

As per the above formula, the direction of force is dependant on the nature of charge as well as the direction given by the cross product. If we consider the charged particles to move with uniform speed, the net force acting on the charge must be equal to zero. I think we must neglect the gravitational force exerted by Earth on the charges since the mass of charges is negligible. The two forces, Centrifugal force and the magnetic force (exerted by Earth's magnetic field due to the motion of the charged particles) cancel each other when they have the same magnitude but opposite direction. From the formula, it is also evident that the direction of force changes with the sign of the charge. It must be opposite for positive and negative charges.

So, based on this is it correct to assume, if the positive charges orbit the earth in one direction the negative charges orbit the earth in the opposite direction? Do they really orbit Earth in opposite directions?

The following image (a view from the top of poles of Earth) depicts my idea on the sense (clockwise or anticlockwise) of rotation of charges in the Van Allen Radiation Belt:

Image Source : My own work :)


texts: Can gravity cause a Big Crunch, if the cosmological principle holds?

texts:
Say we have a closed universe that follows the cosmological principle. As a result, mass is distributed equally and, if as far as I know, gravity should have no net effect on the movements of galaxies (as in, there are no peculiar velocities).

However, in the Big Crunch, the Universe stops expanding and gravity causes the scale factor to decrease. How is this possible? Shouldn't gravity have no effect on a homogenous and isotropic universe?

On that note, if gravity actually caused a collapse, would gravity cause the comoving coordinates of the galaxies to change?


texts: Comparing orbits between a planet and Red Dwarf

texts:
I have a MATLAB problem, so I have an earth like planet orbiting around a sun like star and then there is a red dwarf on an elliptical orbit around the star that passes close to the planet. I want to show that sometimes when the Red Dwarf is at it's closest approach the planet may not be there so it won't gain any heat from the red dwarf.

To do this I was told to compare the orbital periods of the two. So for the planet it's orbital period is just like the earth 365 days and the red dwarf's period is 1896.59 days.

I was thinking is there a way I could plot the orbits of these two and run it for say like 100 years and see the if there is points when the earth could miss the red dwarf? If this is wrong please let me know a better way of tackling this problem.
Below is an image of the orbit, with the black line being the red dwarf's orbit and the  blue line being the planet's orbit. The eccentricity of the Red Dwarf's orbit is 0.866 since a=3 Au and b=1.5 Au.


texts: Meteor traveling through atmosphere without hitting Earth

texts:
Is it possible (however unlikely) to have a meteorite miss Earth so narrowly and at such flat angle that it would enter atmosphere, travel few (dozens, hundreds, thousands?) kilometers getting as close as few kilometers to the ground without hitting anything and then leave atmosphere without any significant, leaving millions of humans staring agape at the sky?

Or does gravity and air friction prevent this by either making such flat trajectory impossible or by slowing down meteorite enough to make it fall to the ground?


texts: Are gas giants supported by thermal pressure?

texts:
I've heard gas giants are supported because there is an equilibrium between thermal pressure and gravity. That is, if Jupiter were to suddenly begin compressing, temperatures would increase to the point that it would expand to its original size. Since it is in equilibrium, both the compression and thermal pressure counter one another, so its size never fluctuates.

However, Jupiter's average temperature is extremely low, so much so that its gases are electron degenerate. Is this a contradiction, or has the information above been just hearsay?


texts: Accessing SDSS server SQL server via python

texts:
Is there a way to directly access any of the Sloan Digital Sky Survey servers via SQL via Python?
Currently I have been able to run SQL queries from the website they provide (for example https://skyserver.sdss.org/dr12/en/tools/search/sql.aspx), but it's not clear to me how I can access them directly from a Python interface.
Any suggestions what connection details I would need to use are highly appreciated.


texts: Who discovered the relation between CMB fluctuations and large-scale structure formation?

texts: I haven't been able to find a reference to the first who showed that the small density fluctuations in the cosmic microwave background led to the formation of large scale structure as we know it.


texts: How far away are the events that caused the gravitational waves that have been detected?

texts: A certain number of gravitational wave events have been detected. Is it possible to know how far away the mergers that caused those gravitational wave events are?


texts: What would happen if asteroid 99942 Apophis collides with the earth?

texts:
In the news recently it's been said that in 2029 the asteroid called Apophis, aka 'The God of Chaos', will fly by very close to earth. To quote Geek.com

  In 2029, a massive asteroid called 99942 Apophis will fly past our
  planet at an approximate distance of 19,000 miles within the distance
  of some orbiting spacecraft.

It's said the chances of a collision are about 1 in 100,000, which is small but not not completely infeasible either.

The asteroid has a diameter of around 350 meters with a mass of 4×10^10 kg and an average orbital speed of around 30km/s.

What kind of damage could we expect on earth if it were to collide with us?


texts: Is it "weird" that all disc galaxies rotate once every billion years?

texts:
In a recent paper (Cosmic clocks: A Tight Radius - Velocity Relationship for
HI-Selected Galaxies by Meurer, et al.), it was noted in the conclusion that:

  [This] implies a constant orbital time of ∼1 Gyr at [the outermost] radius [of disc galaxies].

Given what we knew about disc galaxies and dark matter up to this point, is this an unexpected conclusion? My sense is that it is strange that all disc galaxies have a constant rotation regardless of their size, but my intuition may be wrong. I know that dark matter speeds up the rotation of the outer parts of disc galaxies, but isn't it strange that they converge on this value?


texts: Is there any theoretical or empirical research about planetary systems with a black hole in their center?

texts:
I assume such a system would be hard to detect due to a lack of radiation, at least if the vicinity of the black hole does not contain a lot of mass which falls into it over time, thus creating (a lot of) radiation.

Since black hole formation in a supernova explosion is a process which destroys any planetary system around its original star, any planetary system around a black hole must have formed after black hole formation.

A few objections come to mind off-the-cuff: Because black holes have a large mass there is a minimum distance for planetary orbits below which they would disintegrate due to tidal forces. This may interfere with the usual evolution of planets (they clear their orbits and kick out interfering bodies, for example), because their "orbital space volume" becomes too large for them to keep clean, and the orbital period becomes long.

Is anything known or is there any speculation about such systems?


texts: Is metallicity low at the central region or nucleus of the Milky Way? Does metallicity decrease or increase as we move from the center out to the edges of our galaxy?


texts: How do higher numbers affect orbital resonance?

texts: When talking about orbital resonance, the most common resonances in our Solar System are 1:2 (think Jupiter's moons) and 2:3 (think Pluto and Neptune). However, the lack of larger numbers within these resonances confuses me. I accumulated some data about resonances and I figured out that the difference between the numbers gives more stabilized resonances. For example, a 1:29 resonance would not be stable due to the massive separation of the objects from each other and the gravitational meddling of other bodies. However, that also brings up a different question, and the one I will be asking. A 28:29 resonance seems somewhat stable; after all the objects are close together and will have high gravitational influence on each other. Space is big, so it is unlikely the objects would collide, so why do we not see any 7:8 resonances or other resonances involving high numbers in our Solar system?


texts: Are all satellites thought to be the result of collisions with the planets?

texts:
The leading hypothesis for how the Moon was formed was that another object collided with the Earth, throwing off a large lump of matter which formed into the moon.

Is this thought to be the case with all satellites? e.g. Phobos, Deimos, Charon, Titan, Europa, etc. ? If not, then why do we need such a dramatic "origin story" for the Moon but not the others? Why do we not just assume the Moon was formed the same way as all satellites?


texts: What is the explanation for rapid inflation just after the big bang?

texts: I often hear about cosmic inflation occurring rapidly after the Big Bang, but have never came across any explanation for it. What supposedly caused this inflationary period in the early universe? Also, my assumption is that the laws of physics, i.e. light speed and relativity, did not exist during inflation, but were after effects. Is that a correct assumption? Thanks!


texts:
Please see this image:

I wonder two things about these black holes. The first is, what are the oblong, smaller black hole-looking voids outside the perimeter of the major black holes? Also, why are these black holes not entirely circular, given that their mass is concentrated in a point?  Is this just visual distortion?


texts: What is the history of the Milky Way's relationship with Andromeda?

texts:
From what I've read, the Milky Way will collide with Andromeda in about 4 - 5 billion years, and a billion or so years after that the two will merge and become a giant elliptical galaxy.

What I'm wondering is what the history, orbital dynamics wise, of these two galaxies is? What's set them on the collision course that they now find themselves on? How has the distance between the two varied over time? Could past mergers have robbed one of the two or both of the angular momentum necessary to keep them from colliding?


texts: What does a "point spread function" look like for long exposures from the VLT's large interferometric aperture?

texts:
In interferometric radio astronomy UV plots are the first step in understanding what a point spread function (PSF) will look like for a given location in the sky observed over a period of time. The larger the number of dishes used, the better the choice of locations for each dish (pseudo-random spiral-like ground patterns) and the longer the observation (making longer arcs in the UV plane) all help to reduce the size of the PSF and reduce any bright secondary spots within it.

Math behind a uv plot in interferometry?
In astronomical interferometry, what values do the points in the uv-plane have?

Bits of a discussion in comments below the currently unanswered question Are they really sure this isn't an Airy disk? How was that ruled out? goes something like:

I think part of the answer is that interferometric point-spread functions generally do not look like the PSF of single-aperture telescopes, with a circular, symmetric Airy disk.

and

where the image is built up over several nights from several different sets of three1 "holes" rather than interfere them all at once, (they move the telescopes around in between) and I don't know know how they synthesize that to get one image.

Question: So I'm now wondering, with the VLT's large but sparse interferometric aperture (there are only four primary telescopes though I think there are some secondary (auxiliary) telescopes that feed into the interferometry as well): What does a "point spread function" look like for long exposures?

1should be four2 I think, plus those auxiliary telescopes
2No, three was right for that image, hat tip to astrosnapper